1. Field of the Invention:
The present invention relates to a piezoelectric vibration generator having piezoelectric elements attached to opposite surfaces or sides of a resilient plate for generating slight vibratory motion, and also to a vibratory parts feeder incorporating such piezoelectric vibration generator for feeding small parts or articles in a predetermined supplying direction.
2. Description of the Prior Art:
Typical piezoelectrically driven vibratory parts feeders of the type described are disclosed in Japanese Patent Laid-open Publication (JP-A) No. 62-4118, 62-4119 and 62-4120. The disclosed parts feeders include a base and a vibratory conveyor table supported on the base by means of a plurality of properly spaced apart and obliquely disposed piezoelectric vibration generators. Each of the piezoelectric vibration generators comprises a leaf spring secured at a lower end to the base and carrying on its opposite surfaces a pair of piezoelectric elements, and an elongate resonant or amplitude enhancing member extending continuously and upwardly from the upper end of the leaf spring and secured at its upper end to the vibratory conveyor table. The amplitude enhancing member has a stiffness smaller than the stiffness of the leaf spring and hence is bendable easier than the leaf spring. When the piezoelectric elements are driven by an alternating voltage, the leaf spring is caused to shorten and lengthen alternately, thus producing longitudinal vibrations whose amplitude is in turn enhanced by the amplitude enhancing member. The thus enhanced vibratory impulses are imparted to the vibratory conveyor table, causing small parts or articles to move in a predetermined supplying direction along a path defined on the upper surface of the conveyor table. The amplitude enhancing member is formed either as a separate connecting member which is structurally independent of the leaf spring, or as an upper extension of the leaf spring which is integral with the leaf spring.
With the provision of the amplitude enhancing member, relatively large vibratory impulses can be imparted to the vibratory conveyor table. However, the amplitude of vibrations thus obtained is still limited to a certain level as the overall length of the vibration generator is determined depending upon the distance between the base and the vibratory conveyor table. It is therefore likely to occur that a desired amplitude of vibration cannot be produced from the prior art vibration generator. Another difficulty associated with the prior art vibratory parts feeder is that the amplitude enhancing member, which extends upwardly from the upper end of the leaf spring and has a smaller stiffness than the leaf spring, must be rigid enough to withstand the loads imparted thereon from the vibratory conveyor table. Since the stiffness of the amplitude enhancing member is inversely proportionate to the amplitude of vibrations produced from the vibration generator, setting of the amplitude of vibrations requires a fine adjustment of the stiffness of the amplitude enhancing member, which is tedious and time-consuming. Furthermore, the prior art vibratory parts feeder is relatively large in height as the leaf spring and the amplitude enhancing member extend linearly between the base and the vibratory conveyor table.